Olympian Oscar Pistorius, known as the bladerunner, an amputees recognized worldwide for his athletic accomplishments. Now he's one of the world's highest-profile murder defendants. He insists it was just a horrible accident 2that left his model girlfriend reeva steenkamp dead in his home. The trial under way in a south african courtroom. And we welcome ABC's hamish McDonald to "Nightline" with the latest. Reporter: He was there to face justice, but first Oscar Pistorius faced the cameras. Open up. Reporter: The man known as the bladerunner, an international sporting superstar, is on trial for the murder of his girlfriend, model reeva steenkamp, in the early hours of Valentine's day last year. The photograph is admitted but not as a true reflection without being tampered with. Reporter: In court he's escorted past the family of the woman he killed. Ree reeva's mother says she wants to look him in the eye and see the truth. The first count against the accused, count 1, is one of murder. Reporter: Four charges are read to the court, including murder. The accused unlawfully and intentionally did kill a person, to wit reeva steenkamp. How do you plead? Not guilty, my lady. Reporter: He claims it was a tragic accident, THA he mistook his girlfriend for an intruder. But the prosecutor says he will prove Pistorius killed in cold blood. The only inference from the circumstantial evidence, that the accused shot and killed the deceased with direct intent to kill. Reporter: The court proceedings are a far cry from their days as South Africa's most celebrated couple, Oscar Pistorius and reeva steenkamp had that magic formula, success, beauty, and riches. This is the world's first bionic man, who overcame all these extraordinary obstacles, almost a mythlogical character in some ways, and then it tragic event occurs which takes away the life of this gorgeous woman and then has the potential as an event to destroy his life in the process. Reporter: He an inspiration to many. She a model, a law school graduate, and reality TV star. Hi. My name's reeva, and I'm a contestant on "Tropic island of treasure 5." They had charmed each other and they were charming the public. There's just something about her heart. She had this willingness to help people and look after people and make sure that they were happy. Reporter: Just ten days before the shooting the couple is seen here, flirting in a supermarket. As Valentine's day loomed, love was on her mind. She tweeted, "What do you have up your sleeve for your love tomorrow?" But on the night before Valentine's day things took a tragic turn. This is the last known image of ree reeva, which first aired on south african television. She enters Pistorius's gated compound. Ten minutes later he follows in his white BMW. Later that night reeva texts her best friend's family to say, "I'm sleeping at Oscar's tonight." And that would be the last time anyone outside would hear from her. Pair paralympian Oscar Pistorius -- Well-known model had been shot in the head and body. I started crying. I started screaming. I can't even tell you how many times I told reeva I can't live without her. Reporter: There is no question Oscar Pistorius shot and killed reeva steenkamp. At the heart of this trial is why. I was convinced that woman was being attacked. Reporter: The prosecution claims the couple had a loud fight. A neighbor, whose testimony wasn't shown on camera, told the court today she heard a blood-curdling scream from a woman and then four gunshots. But just before the gunshots it was blood-curdling. It was something that leaves you cold. Reporter: Prosecutors say reeva fled to the bathroom during an argument, that Pistorius grabbed his gun and fired through the door, hitting reeva three times, in the hip, elbow, and head. But the defense tells a different story. In a sworn statement Pistorius claims he thought reeva was an intruder and that he feared for his life. This case is captivating south Africa, being compared to the O.J. Simpson trial. This is probably the biggest trial of the century, certainly so far. Reporter: Interest is so high a dedicated cable channel is now on air to cover proceedings. That tension between this almost godlike character, my mythological character, and the awful tragedy of what happened is something no one can resist. Count 1 is one of murder -- Reporter: In a crowded courtroom Pistorius appears almost isolated, a lonely figure. But in that same way we've seen him on the field he's calm and focused, almost as if preparing for competition. It's only a matter of waking up and thinking I can do it. Reporter: Oscar Pistorius was born without bones in his lower legs. After consulting with dozens of specialists, his parents made a decision to amputate from below the knee. He won a gold medal at the Athens parali pair olympics but perhaps his greatest achievement, competing against able-bodied. He was not wearing prosthetics at the time of the shooting a point that may prove crucial during the trial. He owned a small arsenal of weapons and is being charged with weapons-related counts. These new images show Pistorius allegedly firing the same pistol he later killed steenkamp with. And one of the things he would do is say I'm sort of antsy, I can't sleep, let's go to the shooting range. I thought that was somewhat bizarre then, and in the light of day even more bizarre now. Reporter: It wasn't just for sport. He claims he had legitimate concerns about safety. Violence is rampant in south Africa. Oscar has said that there was a great fear factor involved and that could have framed his mind at the time of the incident. And that could have put him in a position where he had an objective or subjective belief that he was in stage and that could affect his defense, which is self-defense. Reporter: His fate will be decided by one well-respected judge, supported by two advisers. The case will focus heavily on forensics. John Cullen is writing a book about the athlete. With some cooperation from his family. This is something which has really traumatized the family as a whole. Pistorius himself. And bear in mind that whatever the outcome of this trial, Oscar Pistorius is condemned to a lifetime of regret. Reporter: Today outside the court the scene was chaos. When the superstar left, the shooting of reeva steenkamp a year ago has meant chaos for two families. I don't have any regrets. I'm going to miss you all so much. I love you very, very much. Reporter: The trial is set down for at least three weeks, but a long list of witnesses means it could last much longer. For "Nightline" I'm hamish McDonald in Pretoria, south Africa. Thanks, hamish. Stay tuned to ABC news for full coverage of the Pistorius trial. Next, there's fierce

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